Computed tomography (CT) scanning and positron emission tomography (PET) scanning are known methods for diagnostic medical imaging. CT scanning employs multiple X-ray images taken in multiple directions (i.e., with a scanner situated in different orientations relative to a patient) to generate a 3-dimensional image or multiple tomographic image slices. PET scanning employs a gamma-emitting radiopharmaceutical ingested by a patient or injected into a patient. Multiple gamma ray images are taken in multiple directions to generate a 3-dimensional PET image or multiple slices. CT and PET scanning provide different information. For example, CT scanning generally has higher resolution and is superior for providing structural data such as the structure of bones, organs, etc. PET scanning generally has lower resolution but provides more useful information regarding the functional condition of body tissues and systems such as the cardiovascular system. PET is superior for indicating the presence of soft tissue tumors or decreased blood flow to certain organs or areas of the body, for example. The complementary strengths of CT and PET scanning can be provided simultaneously by performing both methods in a single apparatus and imaging session. However, combining CT and PET scanning presents technical challenges because CT and PET require different scan times and have different sensitivities to patient motion.
PET scanning requires a relatively long duration data acquisition period, on the order of several minutes (e.g., about 30 minutes) for a typical clinically sufficient image. Typically, a large number of PET data acquisitions are acquired at many different angles during this period. Consequently, patient movement is a problem in PET scanning. Excessive motion of a patient can result in scan failure. Thoracic cage movement caused by breathing is a significant problem in PET scanning. By comparison, CT scanning is relatively fast and can typically be performed during one breath-hold by a patient.
Fusion of CT and PET images (i.e., PET-CT or CT-PET imaging) often is inaccurate because of inevitable patient movement and breathing. Associated problems include several types of CT artifacts, errors in the association between anatomy and PET uptake, motion blur in PET, and quantitative PET errors such as miscalculation of the standard uptake value due to underestimation or overestimation of attenuation.
Gated scanning has addressed some issues related to motion blur. Gated scanning is described in U.S. Pat. No. 8,060,177 to Hamill and U.S. Patent Publication 2013/0085375 to Hamill et al., the entirety of which applications are hereby incorporated by reference herein. Gated scanning (e.g., gated PET or gated CT) involves identifying and exploiting a physiological signal (e.g., respiratory or cardiac signal) that is cyclical in nature. Based on measuring such a physiological signal, the motion of an appropriate organ (e.g., lung or heart) can be determined during an acquisition. This information can be used to detect time intervals (referred to as gates, time gates, or time windows) of relatively little organ motion. Various gating algorithms (various ways to determine the gates) are known. Image creation (reconstruction) can then be restricted to using data (e.g., amplitude data) corresponding to the times within the time gates, such that the time gates are basically filters in the time domain. In particular, one or more time gates may be assigned per cycle of the physiological signal, and similarly situated gates in respective cycles can be used to reconstruct a dataset.
An example of gating is shown in FIG. 1. In FIG. 1, a plot of a cardiac signal 100 includes multiple cycles, where a cycle may be defined, e.g., from one R-peak to the next R-peak. R-peak refers to the R portion of the QRS complex, which is a series of three graphical deflections in a cardiac signal. Eight complete cycles 110-1, . . . , 110-8 are shown in this example. FIG. 1 shows a 4-gated scan example with gates labelled 1, 2, 3, and 4 that span the entire cycle and that do not overlap one another. In other gating examples, gates may not span an entire cycle or may overlap one or more other gates within a cycle. For visualization purposes, gates 1, 2, 3, and 4 are shown in FIG. 1 as rectangles at different vertical displacements (i.e., like an ascending staircase), but the gates may be just the one-dimensional time intervals corresponding to those rectangles. As shown in FIG. 1, the same pattern of gates occurs in each cycle. To avoid visual clutter, the gates are not designated with reference numerals in each cycle in FIG. 1 but are instead labelled only within cycle 110-4. All gates having the same gate number can be used to reconstruct a dataset that has less motion blur than would occur without gating.
Although gating based on a cardiac signal is shown in FIG. 1, gating may be based on other cyclical physiological signals. For example respiratory gating may be a performed with a respiratory signal, for which a cycle may be defined from an inspiration peak to the next inspiration peak.
Even with gated scanning, there remain problems with motion blur, particularly when gated scans of two different imaging modalities (e.g., PET and CT) are used. Additionally, the problem of two separate motions (heart motion due to heartbeat and lung motion due to breathing) causing blur in the cardiac imaging scenario has remained challenging even with gating.